If you prefer to use a web user interface for code reviews, you can now submit
your patches for Clang and LLVM at LLVM’s Phabricator instance.

While Phabricator is a useful tool for some, the relevant -commits mailing list
is the system of record for all LLVM code review. The mailing list should be
added as a subscriber on all reviews, and Phabricator users should be prepared
to respond to free-form comments in mail sent to the commits list.

To get started with Phabricator, navigate to http://reviews.llvm.org and
click the power icon in the top right. You can register with a GitHub account,
a Google account, or you can create your own profile.

Make sure that the email address registered with Phabricator is subscribed
to the relevant -commits mailing list. If you are not subscribed to the commit
list, all mail sent by Phabricator on your behalf will be held for moderation.

Note that if you use your Subversion user name as Phabricator user name,
Phabricator will automatically connect your submits to your Phabricator user in
the Code Repository Browser.

The tool to create and review patches in Phabricator is called
Differential.

Note that you can upload patches created through various diff tools,
including git and svn. To make reviews easier, please always include
as much context as possible with your diff! Don’t worry, Phabricator
will automatically send a diff with a smaller context in the review
email, but having the full file in the web interface will help the
reviewer understand your code.

To get a full diff, use one of the following commands (or just use Arcanist
to upload your patch):

gitshowHEAD-U999999>mypatch.patch

gitformat-patch-U999999@{u}

svndiff--diff-cmd=diff-x-U999999

To upload a new patch:

Click Differential.

Click + Create Diff.

Paste the text diff or browse to the patch file. Click Create Diff.

Leave this first Repository field blank. (We’ll fill in the Repository
later, when sending the review.)

Leave the drop down on Create a new Revision… and click Continue.

Enter a descriptive title and summary. The title and summary are usually
in the form of a commit message.

Add reviewers (see below for advice). (If you set the Repository field
correctly, llvm-commits or cfe-commits will be subscribed automatically;
otherwise, you will have to manually subscribe them.)

In the Repository field, enter the name of the project (LLVM, Clang,
etc.) to which the review should be sent.

Select the review you want to from the Attach To dropdown and click
Continue.

Leave the Repository field blank. (We previously filled out the Repository
for the review request.)

Add comments about the changes in the new diff. Click Save.

Choosing reviewers: You typically pick one or two people as initial reviewers.
This choice is not crucial, because you are merely suggesting and not requiring
them to participate. Many people will see the email notification on cfe-commits
or llvm-commits, and if the subject line suggests the patch is something they
should look at, they will.

Here are a couple of ways to pick the initial reviewer(s):

Use svnblame and the commit log to find names of people who have
recently modified the same area of code that you are modifying.

Look in CODE_OWNERS.TXT to see who might be responsible for that area.

If you’ve discussed the change on a dev list, the people who participated
might be appropriate reviewers.

Even if you think the code owner is the busiest person in the world, it’s still
okay to put them as a reviewer. Being the code owner means they have accepted
responsibility for making sure the review happens.

Phabricator allows you to add inline comments as well as overall comments
to a revision. To add an inline comment, select the lines of code you want
to comment on by clicking and dragging the line numbers in the diff pane.
When you have added all your comments, scroll to the bottom of the page and
click the Submit button.

You can add overall comments in the text box at the bottom of the page.
When you’re done, click the Submit button.

Phabricator has many useful features, for example allowing you to select
diffs between different versions of the patch as it was reviewed in the
Revision Update History. Most features are self descriptive - explore, and
if you have a question, drop by on #llvm in IRC to get help.

Note that as e-mail is the system of reference for code reviews, and some
people prefer it over a web interface, we do not generate automated mail
when a review changes state, for example by clicking “Accept Revision” in
the web interface. Thus, please type LGTM into the comment box to accept
a change from Phabricator.

Once a patch has been reviewed and approved on Phabricator it can then be
committed to trunk. If you do not have commit access, someone has to
commit the change for you (with attribution). It is sufficient to add
a comment to the approved review indicating you cannot commit the patch
yourself. If you have commit access, there are multiple workflows to commit the
change. Whichever method you follow it is recommended that your commit message
ends with the line:

DifferentialRevision:<URL>

where <URL> is the URL for the code review, starting with
http://reviews.llvm.org/.

This allows people reading the version history to see the review for
context. This also allows Phabricator to detect the commit, close the
review, and add a link from the review to the commit.

Note that if you use the Arcanist tool the DifferentialRevision line will
be added automatically. If you don’t want to use Arcanist, you can add the
DifferentialRevision line (as the last line) to the commit message
yourself.

Using the Arcanist tool can simplify the process of committing reviewed code
as it will retrieve reviewers, the DifferentialRevision, etc from the review
and place it in the commit message. Several methods of using Arcanist to commit
code are given below. If you do not wish to use Arcanist then simply commit
the reviewed patch as you would normally.

Note that if you commit the change without using Arcanist and forget to add the
DifferentialRevision line to your commit message then it is recommended
that you close the review manually. In the web UI, under “Leap Into Action” put
the SVN revision number in the Comment, set the Action to “Close Revision” and
click Submit. Note the review must have been Accepted first.

On a clean Git repository on an up to date master branch run the
following (where <Revision> is the Phabricator review number):

arcpatchD<Revision>

This will create a new branch called arcpatch-D<Revision> based on the
current master and will create a commit corresponding to D<Revision> with a
commit message derived from information in the Phabricator review.

Check you are happy with the commit message and amend it if necessary. Now switch to
the master branch and add the new commit to it and commit it to trunk. This
can be done by running the following:

If you decide you should not commit the patch, you should explicitly abandon
the review so that reviewers don’t think it is still open. In the web UI,
scroll to the bottom of the page where normally you would enter an overall
comment. In the drop-down Action list, which defaults to “Comment,” you should
select “Abandon Revision” and then enter a comment explaining why. Click the
Submit button to finish closing the review.

Please let us know whether you like it and what could be improved! We’re still
working on setting up a bug tracker, but you can email klimek-at-google-dot-com
and chandlerc-at-gmail-dot-com and CC the llvm-dev mailing list with questions
until then. We also could use help implementing improvements. This sadly is
really painful and hard because the Phabricator codebase is in PHP and not as
testable as you might like. However, we’ve put exactly what we’re deploying up
on an llvm-reviews GitHub project where folks can hack on it and post pull
requests. We’re looking into what the right long-term hosting for this is, but
note that it is a derivative of an existing open source project, and so not
trivially a good fit for an official LLVM project.